PhD defence: Joan D'Arcy

Joan D'Arcy defends her thesis,

Chromium Isotope Variations - In modern and ancient environments

Supervisor
Professor Robert Frei, IGN

Assessment committee
Professor Tod Waight (chairman), IGN
Professor Rachael James, University of Southampton - UK
Professor Thomas Nägler, University of Bern - Switzerland

Abstract
Chromium (Cr) stable isotopes are a useful tracer of changes in redox conditions because changes in its oxidation state are accompanied by an isotopic fractionation. For this reason the Cr isotope system is being developed as a potential tool for paleo-redox reconstruction. Dissolved Cr in seawater is incorporated into carbonates. Hence, ancient carbonates can potentially record the Cr isotopic composition (δ⁵³Cr ‰) of seawater in the geological past. Reliable application and interpretation of this proxy requires a detailed knowledge about processes that fractionate Cr on the Earth’s surface, and the quantification the Cr isotope composition of major Cr fluxes into and out of ocean.
This thesis adds to the current knowledge of the Cr isotope system and is divided into two studies. The focus of the first study was to determine what processes control the Cr isotopic compositionof river water and to quantify the δ⁵³Cr value of continental runoff into the ocean. The major findings were that river water is characterised by heavy δ⁵³Cr values (+0.1‰ to +1.6‰), while soils are characterised by light δ⁵³Cr values (-0.3‰), relative to the catchment bedrock (-0.17‰ to -0.21‰), indicating that Cr isotopes fractionate during oxidative weathering of the continents. High river water δ⁵³Cr values (+1.6‰) indicate that dissolved Cr has been back-reduced either in the weathering profile or during river transport. The main conclusion from this study is that δ⁵³Cr value for continental runoff is not uniform and is controlled by prevailing redox conditions in the river catchment. This will have to be considered when interpreting the δ⁵³Cr values of ancient marine sediments, at least from coastal setting.
The second study tests the potential of Cr isotopes as a paleo-redox proxy. The δ⁵³Cr value of marine carbonates deposited during the Early Ordovician — a time of known redox instability in ancient oceans – exhibit a significant positive Cr isotope excursion of +0.5‰. This excursion is interpreted as the reductive drawn down of dissolved Cr in seawater in response to the development of a proximal anoxic sink. The positive excursion does not correlated with other redox proxies (carbon and sulphur stable isotopes profiles from the Lower Ordovician), suggesting that Cr isotopes behave non-conservatively and are more sensitive to changes in local redox conditions. These results demonstrate that Cr isotope composition of ancient marine sediments can be a useful tool for understanding the cycling of redox sensitive elements on a local scale.

The thesis is available at the PhD administration office, 04.1.417